1. Field of the Invention
The present invention relates to a cascade bridge-type DC-AC power conversion method and converter device thereof. More particularly, the present invention relates to the cascade bridge-type DC-AC power conversion method and converter device thereof for enhancing the efficiency of power conversion.
2. Description of the Related Art
Recently, several new energy sources (i.e. solar energy, wind energy and fuel cells) have been widely developed in power generation to reduce relying on the power of fossil fuels, the effect of environment pollutions and global warming. Most power generation systems of the new energy sources connect parallel and supply to a power distribution network. However, these new power generation systems supply DC power. DC power generated from the new energy sources must be converted into AC power by a DC-AC power converter, such that the power generation systems can supply high-quality AC power. Furthermore, the DC-AC power converter must be designed with circuitry in enhancing the efficiency of power conversion, reducing the dimensions and lowering manufacture costs.
For example, TAIWANESE PATENT PUBLICATION No. 1337444 entitled “Cascade Power Converter”, U.S. PATENT PUBLICATION No. 20080031014 entitled “AC/DC Converter Comprising Plural Converters” and U.S. PATENT PUBLICATION No. 20050156579 entitled “Multiphase Converter with Zero Voltage Switching” disclose various power converters. The above-mentioned patents and publications are incorporated herein by reference for purposes including, but not limited to, indicating the background of the present invention and illustrating the state of the art.
A conventional DC-AC converter device mostly comprises a bridge type structure including a single phase structure and a three-phase structure which are applied in a single phase distribution power system and a three-phase distribution power system, respectively. A conventional bridge-type DC-AC converter device employs pulse width modulation (PWM) technology to control power electronic switches of the bridge-type DC-AC converter device, thereby generating a PWM voltage. However, these power electronic switches do not have an ideal characteristic, so that the switching operation of the power electronic switches will cause power loss. The switching power loss will reduce the efficiency of power conversion of the DC-AC converter device. The switching power loss is determined by currents in the power electronic switches, switching voltages and switching frequencies of the power electronic switches.
In order to reduce the switching power loss, a switching voltage of the power electronic switch must be selectively reduced. Hence, recently, a multi-level DC-AC converter device was developed in effectively reducing each switching voltage of the power electronic switches to generate an output of multi-level AC voltages and thus to reduce the switching power loss. Moreover, high-frequency harmonics of the multi-level AC voltages are relatively small, so that the capacity of filters located at an output terminal, the power loss of the filters and the amount of electromagnetic interference (EMI) can be reduced.
In general, a topology of the conventional multi-level DC-AC converter device mainly includes a diode-clamped type, a flying-capacitor type and a cascade bridge type. Each type of the above conventional multi-level DC-AC converter devices has its own advantages and defects. For example, the cascade bridge type includes a plurality of bridge-type DC-AC power converters cascaded, such that the topology of the cascade bridge-type DC-AC power converter device is relatively simple while generating the same levels of output voltages. However, the topology of the conventional cascade bridge-type DC-AC power converter device has an unavoidable defect of adopting several DC sources which has no common point to connect to each DC bus of each bridge-type DC-AC power converter. If the multi-level cascade bridge-type DC-AC power converter device is applied in a single new energy source, a series of additional circuits must be added to generate other DC sources required by the rest bridge-type. DC-AC power converters. Accordingly, this results in a complicated circuit and an increase of manufacturing cost. Furthermore, the power electronic switches of each bridge-type DC-AC power converter must be controlled by high-frequency PWM which further results in a complication of the driving circuit and a higher switching power loss. Hence, there is a need of improving the cascade bridge-type DC-AC power converter device.
As is described in greater detail below, the present invention provides a cascade bridge-type DC-AC power conversion method and converter device thereof. A high-frequency bridge-type power converter and a low-frequency bridge-type power converter are connected in cascade to generate a multi-level AC voltage to reduce a capacity of an output filter. Voltages of DC buses of the bridge-type power converters are asymmetric. The bridge-type power converter with a low voltage of a DC bus is controlled by high-frequency PWM, and the bridge-type power converter with a high voltage of a DC bus is synchronously switched by a frequency identical with that of an AC voltage of a distribution power system. Advantageously, the switching power loss is reduced, the driving circuit is simplified, and the efficiency of power conversion is increased in such a way to mitigate and overcome the above problem. In addition, the cascade bridge-type DC-AC power converter device only requires one DC source circuit to connect to a DC bus of the low-frequency bridge-type power converter, and no additional circuit is required to be performed as a DC source to connect to a DC bus of the high-frequency bridge-type power converter.